Australians who use geo-blocking circumvention tools to sneak into the US-only HBO Now video streaming service, to watch popular TV shows such as Game of Thrones, are being told they will soon be cut off.

Multiple messages seen by Fairfax Media show that HBO has been emailing Australians and other people in areas outside of the US saying they will be cut off on April 21.

"It has come to our attention that you may have signed up for and viewed video content on the HBO NOW streaming service from outside of the authorised service area (the United States, including D.C. and certain US territories)," the email says.

"We would like to take this opportunity to remind you that the HBO NOW streaming service is only available to residents of the United States, for use within the United States. Any other access is prohibited by our Terms of Use."

The email goes on to say that if HBO hasn't heard back by Tuesday next week it will deactivate accounts without further notice.